1. Field of the Invention
The present invention relates to image forming apparatuses such as multi-function printers (MFPs), copiers, printers, and facsimile machines.
2. Description of Related Art
Conventionally, in some image forming apparatuses using toner, such as multi-function printers, copiers, printers, and facsimile machines, there are arranged a photoconductive drum and, opposite it with a gap in between, a developing roller. To the developing roller, a so-called developing bias is applied that has a direct current (DC) and an alternating current (AC) superimposed on each other. As a result, charged toner flies from the developing roller to the photoconductive drum, and thereby an electrostatic latent image is developed. The toner image thus developed is transferred onto and fixed to a sheet, and thereby printing is achieved.
Here, to feed sufficient toner to the photoconductive drum, to obtain desired density in the image formed, and to enhance development efficiency, the peal-to-peak voltage of the AC voltage applied to the developing roller may be increased; however, if it is increased too far, electric discharge occurs between the photoconductive drum and the developing roller. When electric discharge occurs, due to a potential change on the surface of the photoconductive drum, the static latent image is disturbed, and the quality of the image formed is deteriorated. The photoconductive drum can have a property such that, depending on the direction in which the discharge current flows, a large current may flow through the photoconductive drum. When a large current flows, the photoconductive drum may suffer damage, such as a minute hole (pinhole) developing in it. Accordingly, the peak-to-peak voltage may be increased, but within the range in which no electric discharge occurs.
Thus, there is conventionally known a developing unit provided with an image carrying member and, opposite it at a desired interval in the development region, a toner carrying member, wherein a developing bias voltage having a DC voltage and an AC voltage superimposed on each other is applied between the toner carrying member and the image carrying member so that toner is fed to the image carrying member to develop an electrostatic latent image, there are provided a leak generating means for varying a leak detection voltage applied between the image carrying member and the toner carrying member and a leak detecting means for detecting leakage, wherein, as the maximum potential difference ΔVmax between the leak detecting voltage and the surface potential of the image carrying member is increased, when the current flowing between the image carrying member and the toner carrying member increases continuously, the leak detecting means recognizes leakage.
Here, as a big factor that determines the potential difference at which electric discharge occurs, the gap length between the developing roller and the photoconductive drum differs from one image forming apparatus to another due to errors in the fitting and arrangement of the photoconductive drum and the developing roller, variations from the ideal shapes of the photoconductive drum and the developing roller, etc. Moreover, the potential difference at which electric discharge occurs varies under the influence of the atmospheric pressure etc. Accordingly, while the magnitude of the AC voltage applied to the developing roller is varied, the discharge start voltage (the peak-to-peak voltage at which electric discharge starts) is detected. Thereafter, based on the peak-to-peak voltage of the AC voltage at the time of detection of electric discharge, the potential difference between the developing roller and the photoconductive drum at which electric discharge starts is grasped. Then, the AC voltage applied to the developing roller is determined such that, at the time of printing, it is slightly lower than that potential difference.
When electric discharge is detected, however, if toner is carried on the developing roller, since toner is insulating, since the thickness of the toner layer changes the gap length, and for other reasons, the peak-to-peak voltage at which electric discharge starts becomes unstable. In other words, every time it is detected and measured, the discharge start voltage varies. Furthermore, the developing roller carries toner to be charged, and, when the charged toner moves from the developing roller to the photoconductive drum, electric charge moves (a current occurs). Thus, distinction from minute electric discharge is difficult, and “electric discharge” may be erroneously recognized to “have occurred.”
As described above, when occurrence of electric discharge is detected with toner carried on the developing roller, many inconveniences result. There also is a problem in precision, accuracy, etc. Accordingly, occurrence of electric discharge may be detected without toner carried on the developing roller; however, when no toner is carried on the developing roller during detection of electric discharge, no toner is fed to the photoconductive drum. Here, the photoconductive drum and the developing roller have, for reasons of manufacture, deviations (variations) from their ideal shapes. Accordingly, so that a state with a shortened gap length may appear, they are rotated during detection of electric discharge. Furthermore, for removal of residual toner on the photoconductive drum, a contact member such as a blade may be provided.
During detection of electric discharge, if no toner is fed to the photoconductive drum at all, no replacement of toner occurs; thus the toner at a tip part of the contact member making contact with the photoconductive drum keeps being rubbed against the photoconductive drum, and the potential may keep rising. If detection of electric discharge lasts long, the potential of the toner may become extremely high, and dielectric breakdown such as electric discharge or leakage may result. When such dielectric breakdown occurs, the photoconductive drum may suffer damage, such as a pinhole, that may lead to a shorter lifetime and degraded image quality, which is a problem.
Incidentally, in some conventional developing units, no member, such as a blade, making contact with the photoconductive drum is used; in some others, during detection of electric discharge (leakage), the photoconductive drum does not rotate; in still some others, whether or not to carry toner on the toner carrying member cannot be controlled. In view of these (facts), it is clear that the above-mentioned problems cannot be solved in conventional developing units.